Its a good hard SF book and I think its stills holds up today the gun IIRC is more a magnetic linear accelerator which are much better on bodies without an atmosphere or one that is at least near vacuum especially where solar power is available.

Robert A. Heinlein was also co-script writer and technical consultant on the 1950 film Destination Moon which is also worth a watch especially considering it was made almost a decade before Sputnik went beep beep at LEO height.

_________________Someone has to tilt at windmills.So that we know what to do when the real giants come!!!!